Modulation system



Dec. 11, 1934. E. s. PURINGTON 1,984,156

MODULATI ON SYSTEM Filed May 18, 1929 2 Sheets-Sheet 2 INVENTOR E.S.PURINJGT 7 ATTTORNEY t as Patented Dec. 11, 1934 UNITED STATES,

MODULATION SYSTEM Ellison S Purington. Gloucester, :Mass-., assignor to John Hays Hammond, Jr.

Application-May 18, 1929, Serial No. 364,222

r1 Claims.

This invention relates to an improved method ofpush-pull modulationwhich is adapted to produce the side bands ofa modulated wave and Itis the object of this invention to provide an-improved system of modulationof carrier frequency currents. i

It is a fur-ther'object of this-invention to provide an improvedmethod of and system for the modulation of high" frequencies by a push-pull arrangement.

It is a still further object of this invention. to produce a system of modulation wherein the ratio of output side band 'power to the square of the input or audiofrequency power is greater than in the ordinary system.

It-is-another-object of'this. invention to provide a system ofmodulation; wherein the ratio of side band power to carrierpower is such that fine ad.- justments of the carrier circuits are not; so necessary' to reduce the carrier sufiiciently below the side-bands. 1

' Still another object of: this invention is to pro.- vide a systemnofi modulation. which: shall be electrically and mechanically simple.

- Itis a further. object: ofl this invention tov provide a system wherein; the elimination of. the

carriermaybe; easily: accomplished by simpleadjustments to. compensate; :for: the inequalities of the circuit constants and: for small and unequalstraycapacitiesawhicln would otherwise affect the balance.

These and other objects will become apparent from; the following specification taken in con nectiom with. the: appended, drawings. I In. system: ofmodulation of carrier frequencies: the audio: or modulation; current is impressed upomthe. grid of" two modulator devices arranged in: push-pull so that they will operate in phase opposition. The carrier" frequency is impressed upon theplatesof the same devices also in. phase opposition; acting. as a supplement to the: plate potential. The high frequency potentials. tobe radiated are obtained. by means of resistors placed. inthe. plate leads between, the plates and the: high frequency source and are those existing from plate of each tube to ground.

Havingthussbriefly-i described my invention, attention. is invited tothe accompanying drawings in. which;

Fig. 1 is a diagram showing: one embodiment of, my'inventiom 1 Such a circuit is for use in. carrier Fig. 2 is a diagram showing myinvention with special balancing arrangements to permit compensating for the inequalitiesfor the electrical constants including the characteristics of the vacuum tube devices; and r Fig. 3- is a diagramshowing the relation between the plate and grid voltages and currents.

Attention is now'more'particularly invited to Fig. 1' which shows an audioinput including inductancelwhich is inductively coupledby means of a push-pull transformer comprising in addition to the primary inductance 1, the secondary sections 2 and 3 which are connected to the grids of the thermionic modulator devices 4 and 5; respectively. The output circuits of the thermionic devices 4 and 5 include the resistances 6 and'7 and thesplit secondary sections 8 and 9, respectively, of a transformer, and the common source of high potential 10. A biasing potential is providedupon' the grids of the modulator tube by means of the biasing battery 11 and the filaments of the devices are heated by the low potential source I2, adjusted in the usual and well known manner. The positive side of the filament is grounded. The high frequency carrier is supplied as an additional potential upon the plates of the'modulator devices from the source 13 through the inductive relation between the inductance 141 in series with said source and thesplit secondary 8-9 mentioned above. The common plate circuit of the two devices includes the condensers 15 and 16 between which the point NE at neutral potential with respect to the high frequency and audiofrequencies. Connected" to the'point N isthe resistmice 17 connected to ground at 18 and across this resistor 17 exists the modulated side band frequency as will be described more in detail hereinafter. The point 19 may be connected to the antenna orother output or load circuit toradiate the voltages builtup across theresistor 17.

Referring now more particularly to Fig. 2, there is shown a similar circuit. However, across the grid circuits of the modulator devices 35 and 36 areprovided the condensers 3'7 and 38, respec- ,tively. The means for supplying the high frequency tothe plate circuit is, however, the same as shown in Fig. 1, although shown as differently arranged in this instance. The modulated side band output, however, is different. The plate circuit of the modulator device 35; in addition to being connected to the high potential source 44 through the resistance 39 and the half of the split secondary 41; is connected to'theplate of the device, 36 through a balancing circuit, including-the condensersAfl .and;48 :betweenwhich areconnected Cit condensers 51 and 60. The condensers 4'7 and 48 are for insulating purposes only to prevent trouble in case 49 and 50 are accidentally shunted. A three plate balancing condenser 50 is provided having one of the stationary plates connected to the common point of condensers 4'7 and 51, and the other to the common point of condensers 48 and 60 and the adjustable plate connected to the common point N between condensers 51 and 60. The balancing condenser 49 has its stationary plates similarly connected to those of condenser 50 but its movable plate is connected to ground. The point N is connected to ground at 62 through the resistor 61, and also to the point 63 which may be an antenna or other absorbing circuit, the

other side of which is grounded. These balancing condensers are for the purpose of giving the point N of Fig. 2 a zero high frequency potential with respect to the ground, as will be explained more in detail hereinafter.

Referring now more particularly to Fig. 3, there is shown a diagram in which the ordinates represent the plate current of a vacuum tube device given in milliamperes and abscissas represent voltages. The lines 1, 2, and 3 represent the vacuum tube characteristics for the different grid bias voltages as indicated thereon and the lines 4, 5, and 6 show the relations of the possible plate voltages and plate currents for 430, 500, and 570 instantaneously applied volts, assuming a plate resistor of 15,000 ohms. The plate voltage is the applied plate voltage diminished by the voltage drop across the resistor 6 or 7. The voltage of the high voltage source in this diagram is taken as 500 volts and the carrier frequency generator increases and decreases this by an amount of 70 volts, whereby the applied voltage varies from 430 to 570 volts. Then with a'normal grid bias of -22 volts, the plate voltage will vary along the curve 2 from 312 to 358 volts. With an applied grid bias of 14 as at one end of the swing due to the audiofrequency input, the plate voltage will operate along the curve 1 and have a range from 264 volts to 306 volts, whereas with an applied grid bias of 30 volts, as at the other end of the swing due to the audioinput, the plate voltage will operate along the curve 3 and vary between 357 volts and 407 volts. As the high frequency varies the plate voltage and current relations between the lines 4 and 6, and the modulation current varies the grid bias between lines 1 and 3, the plate voltage and current will vary along some such line as the dotted line shown.

It is readily seen that the plate voltage swing due to the carrier impressed onto the plate circuit is greater with the high negative grid voltage with high internal plate impedance than at the peak of the audiovoltage with low internal impedance. It will be noted that if the carrier voltage had been impressed in the grid circuit as in the usual manner, the line of operation would have been along the 500 volt line 5, and for this operating point very little modulation would occur. In fact, with this arrangement the bias is substantially correct for undistorted amplification, and subsequent resistance coupled amplifiers of the side band may use the same operating conditions for plate and grid batteries and plate resistors as in the modulators.

It is evident that if the carrier voltage had been inserted in the grid circuit, a much greater bias would have to be used to obtain modulation, and much smaller grid swing would be possible, especially the more so because the grid swing would have to be shared between the two voltag I will now describe the operation of my in-- vention. The input 1 of Fig. 1 or 31 of Fig. 2 is from a low frequency source, usually the output of an audiofrequency amplifier, and this voltage is inserted to the grid circuits of the modulator. As shown, a balanced arrangement is used, making the grid voltages in phase opposition with maximum voltage on one grid when the voltage of the other is minimum. The voltage of the carrier is impressed into the plate circuit in a similar manner. This allows the grid characteristics to be occupied by the audioinput voltage with greater voltage swing than is possible when both voltages are present in that circuit. Further, the line of operation of the plate characteristics is entirely different in nature, since the plate circuit consumes carrier energy rather than producing it. The carrier power is not a consideration, since it is derived in practice from an oscillating circuit rather than a low power line. The plate current passes through the resistors 6 and 7 of Fig. 1 and 39 and 40 of Fig. 2, and during operation this is a composite current with many frequencies present. By symmetry it will be seen that for both the low and high frequency voltages the two plates will be in phase opposition with respect to the ground. Consequently, with equal condensers 15 and 16 from the plates to the point N, this point is average of the two plate voltages and no voltage of either the original frequencies exists across the output impedance connected from the point to ground; For the side bands, however, it is readily seen that the voltages of the plates for these components are in phase and that for them the point N is not a neutral point. This is because the currents produced by modulation are dependent upon the products of the two modulating currents. If for the tube 4 the original currents are assigned positive values, then the resulting current will be a positive value. But because of the phase opposition of the modulating currents in the two tubes, both must be assigned negative values for tube 5. But the detected currents are positive because the product of two negative quantities is positive, the same as the product of two positive quantities. Consequently, the side band voltages exist across the output impedance 17 from which the original frequencies have been eliminated.

In operation, it is often necessary to provide exact balancing of the carrier frequencies to compensate for inequalities of tubes and resistors and impressed plate voltages. The details of the balancing circuits have been shown in Fig. 2. The balancing condensers of the air type may be so arranged that additional capacity may be thrown to one side or to the other. Preferably there should be no capacity when in mid position. Thus, with circuits identical except that the plate capacity to ground of tube 35 is excessive, balance is obtained by turning the condenser 49 to give additional capacity from plate of tube 36 to ground to obtain exact phase opposition of the two plate carrier wave voltages.

Or, if the capacity balance is made correctly, but due to inequalities of tube conductive characteristics, or resistance values or any other inequalities, a different A. C. plate voltage exists from plate of 35 to ground them from plate of 36 to ground, the point N may be made a neutral point for the carrier frequency by changing condenser 50 to alter the relative voltage drops across coupling condensers 51 and 60.

It is, of course, obvious, that with suitable -"monitoring equipment forindicating lack of carrier voltage across impedance 51, the balance may be made by adjustment of the two condensers 49 and ,50 without technical knowledge of the process involved.

two thermionic devices the inputs of which are inductively related to said primary in push-pull and the plate circuits of which are connected "through impedances, and the two secondary halves of a split secondary transformer to a high voltage plate potential, a source of the high frequency to be modulated including the primary of the transformer the secondary of which is the split secondary last mentioned whereby the plate voltage of the two thermionic devices are supplied in push-pull, a common plate circuit including two condensers in series, the common leads of which condensers are neutral with respect to the high and low frequency voltages, a ground connection for the common filament lead of said thermionic devices, and a resistor connected between said common condenser lead and ground across which high frequency voltages of the side band frequencies exist when the high frequency is being modulated and no voltages exist when the high frequency is unmodulated.

2. Means for the modulation of high frequency energy which comprises a source of audiofrequency, two thermionic devices the inputs of which are related to said source in push-pull phase and the plate circuits of which are connected through resistors and the two secondary halves of a split secondary transformer to a high voltage plate potential, a source of the high frequency to be modulated including the primary of the transformer the secondary of which is the split secondary last mentioned whereby the plate voltage of the two thermionic devices are supplied in push-pull, a common plate circuit including two condensers in series, the common leads of which condensers are neutral with respect to the high frequency voltage, a ground connection for the common filament lead of said thermionic devices, and a resistor connected between said common condenser lead and ground across which highfrequency voltages of the side band, frequencies exist when the high frequency is being modulated and no voltages exist when the high frequency is unmodulated.

3. Means for the modulation of high frequency energy which comprises, a push-pull circuit the input circuit of which is supplied by the low modulating frequency in phase opposition and the output circuit of which is supplied by a high frequency to be modulated also in phase opposition acting as an increment and decrement to the high voltage plate potential of the two thermionic devices included in the push-pull arrangement respectively and a balancing circuit adapted to be neutral with respect to said high frequency potentials when considered in connection with the ground and a resistor connected between said neutral point and ground across of high frequency energy which comprises, a source of audiofrequency including the primary of a transformer,

"which the'high side bandfrequ'encies will exist during modulation and no frequencies will exist when the high frequency is unmodulated.

4. A modulating system comprising a pair of similar three electrode vacuum tubes having their filaments connected, sources of plate, grid and filament potentials for said tubes, a source of modulating current, a source of the current to be'modulated, means for impressing the voltages due to the source of modulating current upon the grid to filament branches of said vacuum tubes with substantially equal voltage magnitude but substantially in phase opposition, 'means for impressing the voltages due to the source of current to be modulated upon the plate to filament branches of said pair of vacuum tubes through substantially equal impedances with substantiallyequal impressed voltages but with phase opposition, a common two terminal output to receivethefuseful output of said tubes,

means for coupling .one of the terminals of said output circuit to the filaments of said vacuum tubes, and means for coupling the otherof the terminals of said output circuit through two similar coupling devices to the plates of said vacuum tubes.

5. In combination, a push-pull modulator circuit including an arrangement of two vacuum tubes characterized by, that the plate to filament voltages are substantially equal but in phase opposition for currents of the modulated and mod-g ulating frequencies, but substantially equal and in equal phase for currents of frequencies produced by the modulation, a two terminal output circuit to receive the products of said modulator and to exclude current of the modulated frequency, means for connecting one terminal of said output circuit to the filaments of the vacuum tubes, two capacity means for coupling the other terminal of said output circuit to the two plates, means for varying the relative magnitudes of said coupling means, and means for varying the relative magnitude of the plate to filament capacities.

6. In a modulation system for obtaining only side bands, a pair of electronic devices each thereof having input and output electrodes arranged in push-pull relationship, a source of modulating energy and a source of carrier energy, means for impressing said carrier energy upon said output electrodes in phase opposition, means for impressing said modulating energy upon said input electrodes in phase opposition, a load circuit, means comprising similar coupling means for connecting one side of said load circuit to the output electrodes of, both said electronic devices and means for connecting the other side of said load circuit to a point of unvarying potential.

'7. In a modulation system for obtaining side bands, a pair of space discharge devices arranged in push-pull relationship each thereof having an input circuit including a control electrode and a cathode and an output circuit including an anode and the cathode, a source of modulating energy coupled to the input circuit of said pushpull arrangement in such a way as to impress upon said control electrodes modulating energy of substantially equal amplitudes and in phase opposition, a source of carrier energy coupled to the output circuit of said push-pull arrangement in such a way as to impress upon said anodes carrierenergy of substantially equal amplitudes and in phase opposition, a circuit for connecting the anodes of said space discharge devices comprising a condenser arrangement having a grounded armature for adjusting the anode capacity to ground of either one of said space discharge devices and a second condenser arrangement connected across said first for adjusting. any inequalities existing due to the space discharge devices, a load circuit, means for comprising similar direct couplings for connecting one side of the load circuit to the two anodes and means for connecting the other side of the load circuit to a point of unvarying potential.

8. In a modulation system, a source of audiofrequency energy, a source of carrier frequency energy, a multi-arm symmetrical electrical bridge structure two adjacent arms of which comprise non-linear variable conductance paths, means for impressing the carrier frequency energy across each conductance path in equal amplitude but in phase opposition means for varying the conductance of said paths in equal amounts but in phase opposition in accordance with the audiofrequency energy, a utilizing circuit and means for connecting the utilizing circuit between a pair of points of said bridge conjugate both to the points across which the audiofrequencyv energy is impressed and the points across which the carrier frequency energy is impressed.

9-. The method of eliminating a carrier and transmitting a modulated side band which comprises impressing high frequency energy difierentially across two paths, non-linearly impeding the passage through said paths differentially, and capacitively coupling a load circuit across said paths.

10.. The method of eliminating a carrier and transmitting a modulated side band which comprises impressing high frequency energy differentially across two paths, non-linearly impeding at audiofrequencies the passage through said paths differentially, and capacitively coupling a load circuit across said paths.

11. The steps in the method of modulating the carrier frequency energy by signal energy, and eliminating the carrier frequency energy from the products of said modulation which comprises impressing carrier frequency energy difierentially across two paths, non-linearly and differentially impeding the passage of the carrier energy through said paths in accordance with the signal energy, and capacitively coupling a load circuit across said paths.

ELLISON S. PURINGTON. 

